In modern printers, such as laser printers, an item to be printed is reproduced as an image on a photosensitive surface. Electrically charged toner is distributed on the photosensitive surface in a pattern that reproduces the original image. The toner pattern is transferred to a receptive substrate, such as paper, in such a manner that the toner pattern reproduces the image on the paper. The toner is then permanently fused onto the paper and subsequently ejected or discharged from the machine.
Curling of the paper occurs during the fusion process. Heat is applied to the image side of the paper at a temperature of over 300.degree. F. and pressure of about 7 kg. The heating operation causes moisture within the paper to dry unevenly such that more moisture is removed from the toner side of the paper than from the other side. This differential drying produces the curling effect.
An additional cause of curling is contraction of the toner particles as the paper cools. The heat generated during the fusion process causes the toner to expand. As it cools, the toner contracts causing the paper to contract and curl toward the toner side. As the paper cools further, curling increases.
Several models of printers incorporate a discharge tray into the shape of the printer case. Printed paper ejected from printers of this type is captured in a discharge tray that is exposed to the heat from the fusion process as heat radiates from the printer. Paper from these printers often is ejected face down so that the toner side is in contact with the printer. Thus, heat from the printer is also transmitted to the paper by conduction. Often these discharge trays have a curved shape. As the ejected paper cools, it tends to reabsorb moisture, curl toward the curved surface of the discharge tray and take on the curved shape of the discharge tray incorporated into the body of the printer.
Printed paper subjected to the above-described curling effect can cause problems when the paper is subsequently fed into other machines such as photocopiers. In addition, curled paper tends to take up more space in file storage cabinets and is more difficult to handle.
Thus, there is a need for an accessory device to certain types of printers to minimize and/or prevent the curling of printed paper ejected from these types of printers.